Heart attacks are a leading cause of death in the industrialized world, particularly in the United States. Many heart attacks are caused in part by a narrowed, stenosed coronary blood vessel. A medical procedure commonly used to deal with coronary vessel stenoses is angioplasty. Angioplasty, in particular Percutaneous Transluminal Coronary Angioplasty (PTCA), includes inserting a balloon catheter into the femoral artery near the groin, and advancing the catheter over the aortic arch and into a coronary artery. The balloon can be advanced through the coronary artery to the stenosis and inflated to widen or dilate the narrowed region. The balloon catheter can then be withdrawn. In some cases, the widened coronary vessel rebounds or re-closes, narrowing the vessel over a period of time.
Stents have come into increasing use to prevent the widened vessel regions from narrowing after angioplasty. A stent, typically having a tubular shape, can be put in place in the widened vessel region to hold the vessel walls apart and the lumen open in the event the vessel attempts to narrow again. One class of stents requires that the stent be forcibly outwardly expanded to put the stent into position against the vessel walls. Another class of stents, self-expanding stents, can be delivered to a site in a compressed or constrained configuration and released in the vessel region to be supported. The self-expanding stent then expands in place to a configuration having a wide lumen, typically pressing firmly against the vessel walls where released. The stent is commonly placed at a recently dilated, stenosed vessel region.
Self-expanding stents can be delivered to a target site mounted over an inner tube or shaft and constrained within the distal end of an enclosing retractable tube or sleeve. The self-expanding stent can be freed from the restraint of the outer sheath by either distally pushing the inner shaft against the stent or proximally pulling the retractable outer sheath from over the stent. Once free of the outer restraint, the self-expanding stent can expand to force itself against the vessel inner walls. Self-expanding stents are often elastically biased to assume an original larger shape after being temporarily compressed into a smaller size to more easily be transported through blood vessels to the target site.
Preferably, the stent is only temporarily compressed within a retractable sheath and compressed for a limited time. The exact size of the stent to be delivered may not be known until the patient is in the operating or treatment room of a hospital. In general, a catheter should have a maximum radial extent or profile no larger than necessary, in part to enable the catheter to reach further into narrower vessel regions. A self-expanding stent is most easily loaded in a proximal direction onto a catheter by compressing the stent and sliding the stent co-axially over the inner shaft distal end and within the retractable outer sheath. The stent must thus typically be slid over the catheter distal tip. The distal tip is optimally tapered, having a proximal width about the same as the width of the outer sheath, to provide a smooth transition from the distal tip to the outer sheath. This can present a situation where the compressed stent has an inner diameter too small to be advanced over the larger outer diameter distal tip of the catheter.
What would be desirable is a delivery catheter and method which would ease loading of a self-expanding stent by not having the tip on the delivery system at the time of loading. This would allow the self-expanding stent to be slid under the sheath with subsequent tip attachment.